1. Field of the Invention
The present invention relates to an optical fiber amplifier to be utilized in optical communication systems which perform wavelength division multiplexing transmission or optical analog transmission, and to an optical amplifier repeater to be utilized in optical communication lines relayed in multistage.
2. Related Background Art
Optical fiber amplifiers to which dynamic energy is supplied in a form of light and which is for amplifying input signal light and emitting the amplified light are mainly used in optical communication systems. Especially, an optical fiber amplifier using an optical fiber in which a rare earth element such as Er is doped is superior in high gain and low noise, and utilized in various ways.
In such optical fiber amplifiers, it is known that gain of the rare-earth-doped optical fiber is wavelength dependence. In particular, in the analog optical transmission, the wavelength dependency of the gain hinders the precise amplification of signal light combined with laser chirping used as a transmitter. In order to improve this point, several attempts have been made to reduce the wavelength dependence.
The first example of these attempts is a technique to reduce the wavelength dependency of the gain by co-doping Al in an amplifying Er-doped optical fiber. This technique is reported, e.g., in "C. G. Atkins et al., Electron. Lett., Vol. 14, 1989, pp1062-1064". The second example of the attempts to reduce the wavelength dependence is a technique of shortening the amplifying Er-doped optical fiber. This technique is reported, e.g., in "S. L. Hansen et al., IEEE Photon. Technol. Lett., Vol. 4, No. 4, 1993, pp409-411".
In a case of long-distance digital optical transmission, an optical amplifier repeater comprising an optical fiber amplifier is used to construct a multistage transmission line in order to compensate optical loss due to transmitting optical fibers. In optical amplifier repeaters, signal light is amplified with the optical fiber amplifier but in this optical amplifier repeater, when the signal light is amplified, noise components distributed in a relatively wide wavelength range including a wavelength at a gain peak are added to signal components with transmission wavelengths of signal light. Here, when the wavelength of the signal light substantially matches with the wavelength at the gain peak of the optical amplifier repeater, the signal components are amplified by a higher amplification factor than the noise components. Consequently, discrimination of signal components and noise components is easy even in the transmission line in which the optical amplifier repeaters comprising the same kind of the optical fiber amplifiers are arranged in multistage.
On the other hand, when the transmission wavelength is different from the wavelength at the gain peak, the noise components are amplified by a higher amplification factor than the signal components. Consequently, discrimination of signal components and noise components is difficult in the transmission line in which the optical amplifier repeaters comprising the same kind of the optical fiber amplifiers are arranged in multistage. Therefore, in the optical communication line in which a plurality of optical transmission lines are connected in multistage through the optical amplifier repeaters to relay and amplify the signal light, a transmitting signal is limited substantially to an optical signal with a single wavelength and a wavelength at a gain peak of the optical amplifier repeater is employed as the wavelength of the signal light.
The wavelength dependency of the gain in the gain operation of the amplifying rare-earth-doped optical fiber depends on glass composition of a core and a type of a rare earth element to be doped. Accordingly, there is a limit on decrease of the wavelength dependency of the gain by co-doping Al in high concentration or by shortening the amplifying optical fiber, and there is also a limit on a wavelength range in which the wavelength dependence can be reduced (T. Kashiwada et al., OAA '93, MA6). In the technique of shortening the amplifying optical fiber, there is also a problem that the sufficient gain cannot be attained.
As the countermeasure for these problems, a passive component (e.g., an optical filter) which has the wavelength dependency of insertion loss to cancel out the wavelength dependency of the gain of the optical fiber amplifier is placed at an output of the optical fiber amplifier. However, with this countermeasure, the loss medium is present, which causes the degrade of energy efficiency used for amplification and which finally causes the degrade of amplification efficiency.
In the optical communication line in which the optical amplifier repeaters are arranged in multistage, generally, the transmission loss between each of the adjacent optical amplifier repeaters is different from others because the distance between each of the adjacent optical amplifier repeaters is different from others. In the amplification operation of the amplifying rare-earth-doped optical fiber, the wavelength at the gain peak is varied depending on the intensity of input light to be amplified. Consequently, in order to make the wavelength at the gain peak of every optical amplifier repeater substantially the same, the wavelength at the gain peak needs to be adjusted for every optical amplifier repeater, or the transmission loss between each of the optical amplifier repeaters needs to be fixed. However, the latter has problems that there is a limit on installation of transmitting optical fibers and that large energy for amplification is needed over the communication line.
As the former method, that is, the method to adjust the wavelength at the gain peak of every optical amplifier repeater, (1) adjusting the length of the amplifying optical fiber, and (2) adjusting the composition of the amplifying optical fiber by co-doping Al are known; however, there is a problem in the method (1) that shortening the optical fiber causes degrade of gain and that lengthening the optical fiber causes degrade of noise characteristic. Consequently, it is difficult to independently control the wavelength at the gain peak. The method (2) can control the wavelength at the gain peak without damaging another properties; however, as shown in FIG. 1, the wavelength at the gain peak is sharply varied at 0-0.5 wt % of the Al-codoped concentration, which causes a problem that precisely adjusting the wavelength at the gain peak is difficult in manufacture. Note that when a plurality of the compositions in which the optical amplifier repeater is placed in the latter stage of the communication line are connected in series, the span loss and the relay amplification are sufficiently stable in the latter, and that FIG. 1 shows a relation between the span loss and the wavelength at the gain peak in this stable condition.